An aerosol is understood to mean particles in liquid or solid phase suspended in air or a carrier gas in the airborne state. The aerosol is considered to be a disperse system formed of solid or liquid particles that are finely distributed in air or a carrier gas.
Aerosols are characterized by single basic features. A single individual aerosol particle is described by three features, specifically shape, size and substance. The aerosol as an accumulation of many individual particles or as a particle collective is described in greater detail by further properties, specifically concentration and particle size distribution.
Optical particle sensors often work with electromagnetic radiations in a wavelength range of from 600 nm to 780 nm.
The wavelength range of 380 nm to 780 nm is also referred to as light, since it lies within the range perceived by the human eye.
Hereinafter, the term “light” will therefore also be used instead of the term “electromagnetic radiation”, since the term “light” includes the range of electromagnetic wavelengths usual for optical particle sensors.
A wavelength of approximately 655 nm is often used, since there are very economical laser diodes with this wavelength as a source for the required light.
In order to measure the particle mass concentration, aerosol photometers (APMs) are used, which are also referred to in the technical literature as “light-scattering nephelometers”.
Aerosol photometers measure the concentration in a particle collective. The measurement result is the particle mass concentration. This is often specified in mg/m3.
Due to their operating principle, aerosol photometers can be used with particle mass concentrations up to several 100 mg/m3.
Either laser diodes or light-emitting diodes (LEDs) are used as monochromatic light source for aerosol photometers. LEDs are used in economical aerosol photometers. In principle, optical smoke detectors for example fall under the group of aerosol photometers.
In the case of aerosol photometers a zero-point adjustment must be made regularly, since contamination and ambient influences lead to a drift of the zero point.
High-quality aerosol photometers are provided with means so as to be able to perform this zero-point adjustment automatically. To this end, the aerosol is firstly guided through a filter or over a separator, so that there are no longer any detectable particles in the measurement volume. The “correction value” then recorded is stored and subtracted from the photometer measured values in the subsequent aerosol measurements. The difference is then output as the photometer measured value.
When it comes to taking ambient measurements in cities, aerosol photometers are suitable measurement apparatuses. In heavily loaded cities, partial particle mass concentrations of more than 0.4 mg/m3 are sometimes measured.
Another class of optical particle sensors is constituted by optical particle counters (OPCs). These measurement apparatuses also use the effect of light scattering in aerosols. However, in contrast to aerosol photometers, it is not a particle collective that is measured, but instead individual particles. To this end, the optical and electrical requirements are much higher than in the case of aerosol photometers. In the case of an aerosol photometer, the light scattered by thousands of particles is detected. Since, in the case of an optical particle counter, only the light scattered by an individual particle is detected, a much higher sensitivity and/or light intensity is necessary.
The optical measurement volume, which in aerosol photometers can easily be several 100 to 1000 mm3, has to be made much smaller in the case of optical particle counters. If, for example, 1000 particles per cm3 are to be measured error-free with an optical particle counter, the optical measurement volume must be only approximately 0.5 mm3 in size. It is thus ensured that only one particle is ever located in the optical measurement volume, up to a particle number concentration of 1000 particles per cm3. There are approximately 1000 particles per cm3 for example in Shanghai with a PM2.5 air load of 120 μg/m3.
At higher particle number concentrations, what are known as coincidence errors occur.
There are then a number of particles simultaneously in the optical measurement volume. These particles are then detected as an individual particle and are classified in an incorrect size class. This produces errors in the measurement result.
These coincidence errors mean that, in the above-mentioned case, this optical particle counter can no longer be used already from a relatively low load of 120 μg/m3.
Optical particle counters, however, do have some technical advantages with regard to their usable concentration range compared to aerosol photometers.
Optical particle counters do not have a zero-point drift, since a signal shape is assessed rather than a signal value.
Besides the number of particles, the particle size distribution (PSD) can also be detected on the basis of the signal shapes.
Optical particle counters calculate the particle mass concentration in an aerosol by dividing the detected particle sizes into size classes (bins) and measuring the frequency of occurrence for each size class or for each bin. Each size class or each bin is assigned a specific weighting factor, which, multiplied by the frequency of occurrence, gives the particle mass for this size class or for this bin.
If the particle masses of all relevant size classes or bins are added together, this gives the total mass concentration.
In order to calculate PM2.5, the particle masses of all size classes or bins up to a particle size of 2.5 μm diameter are added together.
In order to calculate PM10, the particle masses of all size classes or bins up to a particle size of 10 μm are added together.
Optical particle counters respond in a much more robust manner to changes to the particle size distribution in the aerosol. If the particle size distribution in the aerosol changes towards large particles, the mass is underestimated with aerosol photometers, since the mass of a particle grows with the square of the surface area. The scattered light, however, is linear to the surface.
In the case of optical sensors and particle sensors, there is always a risk of contamination. Aerosol deposits on optically effective surfaces cause changes to the properties thereof. In the case of optical particle sensors, not only do such surfaces include the surfaces of the optical components, but also the surfaces of the measurement chamber and beam dump. In principle, dust particles or other contaminations can become deposited on the above-mentioned optically relevant surfaces and can thus falsify the measurement result of optical particle sensors.
Various methods, processes and procedures are known for reducing this risk of contamination.
For example, the aerosol is conducted through a coarse filter or over a course separator prior to passing through the particle measurement apparatus comprising the optical particle sensor. This, however, results in the need to regularly change or regularly clean this course filter or course separator.
In order to prevent or reduce the contamination of the optics, an additional, filtered airflow is generated as a sheath flow and insulates the optics from the airflow loaded with particles. This is a technically complex approach, which is generally only used in correspondingly high-quality particle measurement apparatuses. Here as well it is necessary to regularly change the filters for the sheath flow. In addition, it is not possible to prevent contaminations that are caused by outgassing, for example of plasticisers formed of plastic components, and that are produced fundamentally during the time in which the particle measurement apparatus is not being used.
In addition, it is possible to coat the optical parts and components with dirt-deflecting nanostructures. The contamination is thus indeed significantly reduced, but is not prevented in principle.
In the automotive field, particle sensors with operating times of up to 15,000 hours are expected. The maintenance of these particle sensors must be limited to a minimum. Typical intervals are for example every two years or every 30,000 km.
Particularly in the case of particle sensors, however, it is very difficult to define a fixed servicing interval. Here, the actual operating conditions of the particle sensor play a large role. If the motor vehicle for example is used in unusually heavily loaded areas, for example in Beijing, it should thus be anticipated that the particle sensor will need to be maintained or serviced much more frequently than in other areas loaded to a lesser extent. The moisture of the medium carrying the aerosol to be measured, in conjunction with the particle load also has a significant influence on the contamination of optical particle sensors. There is thus always the difficulty of carrying out a servicing or maintenance operation at the appropriate time. In addition, due to the temperature differences in a car between the temperature in a measurement chamber and the temperature of the drawn-in airflow, it is possible that optically relevant surfaces will be subjected to condensation. It must be possible to identify this particular case of contamination, since the obtained measured values will not be meaningful.